1. Field of the Invention
This invention relates to a radio communication apparatus used in a cellular mobile radio communication system, such as a mobile telephone system, a hand held phone system, or a cordless telephone system, and more particularly to a digital radio communication apparatus having the function of measuring the reception signal strength indicator (RSSI) information of radio channels.
2. Description of the Related Art
In recent years, a digital scheme has been replacing an analog scheme and getting dominant in the field of cellular mobile communication systems.
The digital scheme is such that in the transmission apparatus, the audio signal and data are coded and the carrier is modulated digitally by, for example, a QPSK (quadrature phase shift keying) scheme using the coded signal, and then the modulated carrier sent from the transmission apparatus is received by the reception apparatus, in which the received signal is demodulated digitally, and thereafter the demodulated signal is decoded to reproduce the audio signal and data.
Furthermore, many digital cellular mobile radio communication systems have used a time-division multiple access (TDMA) scheme as a radio channel accessing method. The TDMA scheme is such that a plurality of stations transmit signals using the same carrier in such a manner that the signals may not overlap each other in time. FIG. 7 shows an example of a frame format in the TDMA scheme. In each of an up carrier and a down carrier, a single frame consists of six time slots. When a mobile station gets into communication with a base station, a pair of empty time slots is selected from the six pairs of time slots in the half rate transmission mode, whereas two pairs of empty time slots are selected from the six pairs of time slots in the full rate transmission mode. Then, the selected slot pairs are assigned to the mobile station as radio channels. FIG. 7 shows an example of allocating time slots in the full rate transmission mode. The shaded portions ST1, SR1 and ST4, SR4 indicate the assigned slots.
This type of system provides what is called MAHO (mobile assisted hand-off), which is such that when the quality of the radio channel has deteriorated during communication, the radio channel is changed to another radio channel. FIG. 8 illustrates the operation sequence of MAHO.
The mobile station always measures the RSSI information of the radio channel now in use and the other radio channels and the bit error rate (BER). The measuring of the RSSI and BER of the radio channel now in use is done in a reception slot period SR1. The measuring of the RSSI and BER of the other radio channels is effected in an idle period I excluding reception slot period SR1 and transmission slot period ST1. It is assumed that in this state, the base station has sent a measurement instruction to the mobile station now in communication. Then, the mobile station returns a response to the instruction and then reports the latest measurement data on the RSSI and BER obtained at this time to the base station. Receiving the report of the measurement data, the base station instructs the mobile station to end the measurement. After the mobile station has returned a response to the end instruction, the base station judges the quality of the radio channel now being used, on the basis of the measurement data. If the quality of the radio channel currently being used has deteriorated below a specific level, the base station will instruct the mobile station to hand off the channel to another empty channel. In response to the hand-off instruction, the mobile station hands off the channel to a new radio channel and thereafter communicates with the base station through the new radio channel. Therefore, for example, even if the mobile station has moved to another cell during communication, it can continue communication.
When MAHO is performed, however, conventional radio communication apparatuses have caused the following problem. In a radio communication apparatus used in a digital mobile radio communication system, an AGC circuit is provided in the reception system. The AGC circuit functions so that the received signal may not be saturated even when a high-level modulated signal has been received, and thereby enables the amplitude information component of the QPSK modulated signal to be demodulated accurately. For this reasons the time constant of AGC is set at a value (e.g., 400 msec) large enough to prevent the amplitude information component of the QPSK modulated signal from being followed up.
In a radio communication apparatus using the TDMA scheme, the AGC circuit is brought into an operating state only in the reception slot periods SR1 and SR4 of its own station and is out of operation in the other periods as shown in FIG. 7. Since the AGC circuit has a small time constant as indicated above, it holds AGC value at the end of the reception slot and it starts operating with this AGC value when the next reception slot assigned to the station is supplied to the station. During any other channel period, the AGC circuit remains inoperative. This is because the AGC loop cannot operate as fast as the AGC circuit during this period since the RSSI-measuring time is much shorter (about 2 msec) than the AGC time constant. Because of this, the conditions for the measurement of RSSI in the reception slot periods SR1, SR4 differ from those for the measurement of RSSI in the idle period I, depending on the on/off operation of the AGC circuit, with the result that RSSI cannot be measured accurately.